The present invention relates to an image forming apparatus such as an electrophotographic copier, and an electrostatic printer, and specifically to an image forming apparatus having a magnetic brush type charging apparatus by which an image forming body is uniformly charged.
In a conventional electrophotographic type image forming apparatus, generally, a corona charger is used for charging an image forming body such as a photoreceptor. The corona charger impresses a high voltage upon a discharging wire in order to generate a strong electric field around the discharging wire, and to carry out gas-discharging. The image forming body is charged when ions, generated at the aforementioned operations, are attracted by the image forming body.
As described above, the corona charger which is used for the conventional image forming apparatus can charge the image forming body without mechanically contacting therewith, and therefore, the corona character has an advantage in which the image forming body is not damaged through the charging operation. However, a corona discharger also has disadvantages in which: there is danger that an operator receives an electric shock because high voltage is used in the charger; there is also danger of electrical leakage; ozone generated during gas-discharging is harmful to humans; and the corona discharger shortens the life of the image forming body. Further, the charging voltage potential from the corona charger is unstable because it is strongly affected by temperature and humidity. Yet further, in the corona charger, noises are generated by high voltage, which is a large disadvantage when an electrophotographic image forming apparatus is used as a communication terminal or an information processing apparatus.
The above-described disadvantages of the corona charger are attributed to gas-discharging during charging of the image forming body.
In this connection, the following charging apparatus has been disclosed in Japanese Patent Publication Open to Public Inspection Nos. 133569/1984, 21873/1992, and 116674/1992, in which a magnetic brush is formed by attracted magnetic particles on a cylinder in which a magnet is housed, and charging is carried out when the surface of the image forming body is rubbed by the magnetic brush, as a charging apparatus in which high voltage gas-discharging as in the case of the corona charger is not conducted; the image forming body is not mechanically damaged; and the image forming body can be charged.
However, in the charging apparatus disclosed in the above publications, the following problems have been caused: the image forming body is damaged or uneven charge occurs when the magnetic particles are moved and deposited onto the image forming body at the start and stop of the charging operation. Further, when toner enters into the charging apparatus, the charging capacity is lowered, so that problems of deposition of magnet particles have occurred.
An object of the present invention is to solve the above-mentioned problems and to provide an image forming apparatus with a magnetic brush type charging apparatus in which deposition of magnetic particles onto the image forming body does not occur, no ozone is generated, and stable and uniform charging can be carried out.